Tools

Many browsers support MathML and support in math&science applications is ubiquitous. To write MathML, you don't need more than a text editor, but there are many tools available that make it even easier.

This page of the MathWG's public wiki keeps a non-exhaustive list, including browser, JavaScript polyfills, authoring&conversion tools and more general MathML-enabled software.

Of course, nearly all software has bugs and some programs are further ahead implementing MathML 3 than others.